This invention relates to the alignment of a mirror in an optical system, as well as figure sensing in the optical system, such as an astronomical telescope, by use of optical elements disposed on a reflective surface of the mirror to develop alignment beams of light and, more particularly, to the construction of the optical elements as holographic optical elements (HOE's) by fabrication of each holographic element as a separate assembly of transparent substrate with zone-plate surface features isolated from a reflective coating of the mirror, which coating may be renewed periodically for counteracting weathering of the coating.
Telescopes employed for viewing stars and other distant objects are constructed frequently with a large primary mirror for gather light from the subject being viewed. In one form of construction employing an actively controlled mirror, the primary mirror is formed with a relatively light weight supporting structure which allows for a relatively small amount of deformation of the mirror reflecting surface. While the deformation is small, it is enough to degrade an image obtained by the telescope. The image is corrected by computer reconstruction employing actuators which support the primary mirror or a separate active mirror. A test light beam is used to develop numerous local sample beams diffracted from the holographic optical elements and directed to numerous detectors to check the alignment or deformation of the primary mirror. Any deformation or misalignment of even a portion of the mirror introduces a deflection of a plurality of the beams, which is sensed by the detectors so as to describe the actual mirror reflecting surface. This information allows a computer to correct the resulting image to compensate for effects of the mirror deformation, thereby to provide an accurately formed image of a target. One such optical surface and alignment sensing system is known as the Hartmann system.
A problem arises in that the telescope is often employed in an outside environment wherein weather may produce a degradation or deterioration of the metallic reflecting coating of the mirror. This defective coating is corrected periodically by stripping it from the mirror and applying a new reflective coating to the front surface of the mirror. However, in the Hartmann figure sensing system, the conventional approach is to construct the optical element which generate the sample beams as holographic optical elements by etching zone plate features directly into the reflecting surface of the mirror. For example, the holographic optical elements may be etched directly into the surface or coating of an optic, for instance, into a layer of chromium directly underneath the final reflective metal layer of the mirror. Subsequently, during a refurbishing of the mirror by the aforementioned stripping and reapplication of the coating, the holographic optical elements are damaged, or even totally eradicated, and must be reconstructed by an undesirably complex and costly process of wet lithography for contact printing of the zone plate features.